In the prior art, a fiber grating is formed by an optical fiber to serve an optical fiber sensing function. According to different characteristics, shapes, uses and purposes of measuring points, a fiber grating may made into various structures to form excellent strain sensing elements that accurately measure strain values by means of an externally connected optical wavelength scanning apparatus. A fiber grating is also referred to as a Fiber Bragg Grating (to be referred to as an FBG), which may serve as a fiber grating sensor. In an FBG, the optical fiber is exposed by using a coherent laser, such that the index of refraction of the core of the illuminated section of the optical fiber is permanently changed, and that section of index of refraction of the optical fiber, also referred to as a fiber grating or an FBG, has bright and dark periodical striped intervals A. The manufacturing method of an FBG is as shown in FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D. Referring to FIG. 1A showing a sectional view of a common single-mode optical fiber, a bare optical fiber having an outer diameter of 125 μm is coated by plastic or resin to form an optical fiber having an outer diameter of 250 μm, and 101 represents a 125 μm bare optical fiber surrounding the resin region. In FIG. 1B, the 125 μm outside coating resin around the bare optical fiber is removed to prepare for manufacturing the fiber grating. FIG. 1C shows a manufactured fiber grating from the 125 μm bare optical fiber that is not coated by the resin. In FIG. 1C, 102 represents the optical fiber core, and 103 represents the section of fiber grating having a refractive index with bright and dark periodical striped intervals A. In FIG. 1D, the 125 μm bare optical fiber region of the fiber grating is re-coated by the resin to form a fiber grating having an outer diameter of 250 μm, and 104 represents the 250 μm outside diameter re-coating resin.
In an FBG by using a feedback effect generated by Bragg diffraction, a predetermined wavelength satisfying a Bragg condition, referred to as a feedback Bragg wavelength λB, is reflected in a direction reverse to the incident direction back to a scanning apparatus that emits lightwaves for further analysis, so as to measure whether a received wavelength is increased/decreased. The feedback Bragg wavelength λB is represented by an equation:λB=2nΛ  (1)In equation (1), Λ is the period of the FBG, and n is an effective refractive index of the optical fiber. When the strain is generated in the fiber grating by an external force received, a variance in the original interval Λ is ΔΛ, which is substituted into equation (1) to obtain:λB=2nΔΛ  (2)According to the definition of the strain ε, the gauge length of the force receiving object is set to 1, and Δ1 is the length change due to the force received.ε=Δ1/1=ΔΛ/Λ  (3)It is then obtained that:Δ1=(ΔΛ/Λ)/1=(ΔλB/2n)/(λB/2n)1Therefore:ε=Δ1/1=ΔλB/λB  (4)
Hence, the variance in the reflected Bragg wavelength λB caused by the slightly increased length of Δ1 generated from applying stress on the optical fiber having 1 gauge length is ΔλB. At the light transmitting end, i.e., the feedback reflecting end, a wavelength drift ΔλB in λB is received. In other words, when the wavelength drift ΔλB is received as a force is applied on an optical fiber sensing element, it means that the variance increased in the optical fiber sensing element is Δ1 in length. Such may be utilized to measure whether the force, received by an optical fiber sensing element fixed in parallel by a gauge length of 1 on an object under test, causes a variance s measured during engineering strain to exceeds a limit. A breakage warning signal may be issued if the limit is exceeded. However, because physical properties of an FBG are affected by temperature changes, the wavelength drift ΔλB is also affected. Thus, when an FBG is applied as a sensing device, multiple sensing devices, placed next to one another or closely connected in series, are utilized to obtain reference values of the temperature changes, so as to further perform temperature compensation to correct the accuracy. Alternatively, a fiber grating placed in an optical fiber sensing element is implemented by a chirped fiber grating (CFG) structure. Thus, a dispersion effect is eliminated by two wavelengths (long and short) to overcome the issue of one single sensing element accuracy and to stay unaffected from the temperature.
As previously described, according to different characteristics, shapes, uses and purposes of measuring points, a fiber grating may made into various structures for optical fiber sensing. These structures are fixed in parallel on an object under test to measure a variance ε caused by a force received during engineering strain.
One common features of all conventional structures is that, a predetermined micro strain, e.g., −2500 μs, needs to be tensed to serve as a future tolerable compression amount after the sensing elements are placed at fixed sensing positions, or else an FBG without tensing may easily generate strain hysteresis and lose its accuracy. Thus, ideally, a fiber grating needs to be pulled to a starting wavelength of the measurement, and a minimum tolerable strain value with which the fiber grating can be measured also needs to be achieved. However, from aspects of engineering applications, it is impossible simulate conditions where an enormous object under test in normal use first is compressed, the fiber grating is then fixed, and the measurement is carried out after such compression is released to return to a normal state. Hence, one common feature of all conventional structures is that, a fiber grating is tensed to first obtain a tolerable compression amount of a measuring region.
In the manufacturing process or method of tensing a fiber grating to a tolerable compression of a measuring region, one of two ends of a manufactured FBG is first fixed to a starting point of a gauge length L of an object under test, and a force is applied on the other end of the FBG to pull the FBG to a wavelength of a set tolerable compression strain and then fixed to an ending point of the gauge length of the object under test. Thus, the fiber grating having a gauge length of the two fixed points is tensed, which is equivalently realizing the technology of forming a future maximum tolerable compression amount on the object under test in advance. Such simple tensing operation on one fiber grating is often applied to objects under test having different structures. To achieve an object of the same predicted maximum tolerable compression amount (e.g., −2500 μs), many different tensing methods, tools, parts and operations have been developed, hence complicating an originally simple tensing process of an FBG. These complicated manufacturing operations are quite costly and need to be standardized as well as simplified to reduce costs and to obtain a more accurate tensing amount. That is to say, a first drawback of a conventional FBG is that, although a conventional FBG needs to be tensed, there are no standardized and simple tensing carriers. That is one reason why current FBGs have not yet become standard optical sensing elements.
Further, conventionally, when one FBG is manufactured from one single-mode optical fiber, the structure is usually merely one bare fiber grating having an outer diameter of 0.125 mm (125 μm), and is prone to breakage and cannot withstand lateral pressure. Thus, acrylic or resin for protection is usually coated around the FBG to become an optical fiber having an outer diameter of 250 μm. However, the FBG and an externally connected optical fiber are still likely damaged by external forces, as the conventional fiber grating sensing element fixed on an object under test in FIG. 2. In FIG. 2, 201 is a 125 μm bare fiber grating, 202 is resin coating protection layer, 203 is an FBG region at the 125 μm optical fiber core, 204 is a resin re-coating protection layer, 205 is an PE outer jacketed material layer having an outer diameter of 0.9 mm, 206 is a fixing seat or a fixing adhesive of the FBG, and 207 is a fiber grating carrier or a structure of an object under test. In a conventional method for protecting an FBG, one layer of PE outer jacketed material having an outer diameter of 0.9 mm is added again around the fiber grating. To increase the strain sensitivity of a fiber grating having a section of about 10 mm, in a first method, at this section without the protection of the PE coating material having an outer diameter of 0.9 mm, two ends of the fiber grating are directly fixed to an area of the object under test of 207 by an adhesive. In a second method, two ends of the fiber grating are first fixed to a material same as that of the structure under test or a metal material to form a sensing element. This material same as the material of the structure under test or the metal material combined with the FBG is referred to as a fiber grating sensing element carrier, as 207 in FIG. 2. Two ends of the sensing element are fixed in parallel on the area under test, and the strain sensitivity accuracy is directly or indirectly obtained by a parallel measuring method within a linear elastic limit. As the outer coating material does not have a linear strain characteristic of elastically stretching within an elastic region (i.e., within the elastic limit), to prevent a drawback of plastic deformation caused by a tensile strength exceeding the endurance of the plastic coating material, the outer coating material does not cross the FBG sensing grating region. Such exposed fiber grating without protection, e.g., the region 208 that is an exposed and unprotected fiber grating region, is frequently one of the main reasons causing damage and breakage during the tensing operation, on-site installation process and a measuring process after the installation. Therefore, a fiber grating without the protection of outer coating material is a second drawback that sets back the standardization of optical fiber sensing elements.
In an on-site environment, when the coated optical fibers connected from two ends of the unprotected fiber grating region and having an outer diameter of 250 μm are further connected to external optical fiber segments, the optical fiber at these two ends is extremely slim and cannot be clearly observed, and are thus likely touched to result in optical power loss or breakage. The optical fibers externally connected from the two ends of the fiber grating region are unprotected by an appropriate material and are thus easily damaged or broken—such is a third drawback in standardized optical fiber sensing elements. Thus, to be distinguished and innovated from the technology of a single-mode optical fiber optical cable coating material, and in order to perfectly present a technology that satisfies both optical transmission optical cable technology and standardized optical sensing elements, a structure manufactured by one-time completed process that is an optical fiber sensing cable product formed through integrating a conventional single-mode optical fiber and a fiber grating needs to be provided.